Heretofore, as a filler for a rubber having a high reinforcing effect, a carbon black of an ISAF class is well known. However, in the pneumatic tire for heavy load in recent years, the carbon black which can improve wear resistance has been desired in accordance with demands of improved case durability, resource saving and good mileage.
Thus, the carbon black which can be used as the filler in the rubber composition for the tread of the tire for heavy load has also changed from the ISAF class to an SAF class.
By the way, in order to improve the wear resistance of rubber products such as the tires, it is already known that a high structure carbon black having a fine particle diameter is suitable.
The diameter of the fine particles of the carbon black can be determined by measuring the particles in an electron micrograph, but the thus measured particle diameter is an average value and in fact, it has a certain distribution.
Alternatively, the particle diameter can also be calculated by measuring the surface area of the carbon black, and for this measurement, an N.sub.2 SA method for determining from the amount of an adsorbed nitrogen gas and an iodine adsorption method (an IA method) are simple. These methods have been routinely employed in a quality control of a maker, an acceptance test of a user, and the like.
In measuring the surface area, a point to which much attention should be paid is whether a value obtained by the measurement principle denotes a whole surface area or an external surface area.
It is considered that small pores are present on the surfaces of the carbon black, and the whole surface area also includes the surface area in the pores, but the external surface area does not include the surface area in the pores. When the carbon black is kneaded as a reinforcing agent with a rubber, such giant molecules as in the rubber cannot get into the pores, and for this reason, the surface of the pores cannot be effectively utilized in practice.
Therefore, the external surface area of the carbon black is also called a rubber effective surface area sometimes. In order to determine this rubber effective surface area, there is used a surface area calculated from an amount of adsorbed cetyltriammonium bromide (CTAB) having a large molecular weight. This is called a CTAB method surface area. This CTAB method surface area has a unit of m.sup.2 /g, and it can be measured in accordance with ASTM D-3765-89.
On the other hand, the particles of the carbon black are not present singly but integrally like bunches of grapes, as seen from an electron microscope. This is called a structure. This structure can be classified into two kinds, and one is established by the mutual fusion of the particles, i.e., a chemical bond and the other is established by a physical bond such as a van der Waal force. The former non-destructive structure morphology is called a primary structure, and the latter deformable and destructive structure morphology is called a secondary structure. The structure can usually be evaluated by an absorption value of dibutyl phthalate (DBP). This principle is based on a phenomenon that the carbon black having high entanglement properties among the particles can absorb a larger amount of an oil.
A 24M4DBP absorption value is a value obtained by repeating a compression operation at a pressure of 24,000 psi four times and then measuring a DBP absorption value in accordance with ASTM D-3493, and this absorption value is an index for evaluating the skeleton structure properties of the carbon black mainly comprising the primary structure, used in measuring the DBP absorption value, on the basis of the non-destructive real structure morphology (the primary structure) to the exclusion of the DBP absorption value by the deformable and destructive structure morphology (the secondary structure) formed by the so-called van der Waal force.
In order to improve the wear resistance by adding the carbon black to the rubber, it is required that the carbon black is brought into fine particles. However, if the carbon black is pulverilzed more finely than in SAF to form a higher structure, the dispersibility of the carbon black deteriorates noticeably in the blended rubber. Therefore, such an improvement of the wear resistance as to be expected cannot be observed, and nowadays, the improvement of the wear resistance has already reached a substantial limit. Furthermore, if the carbon black is pulverized more finely than in SAF to form a higher structure, the viscosity of the unvulcanized rubber becomes very high, so that workability deteriorates noticeably.
On the other hand, as a method for improving the wear resistance without using the carbon black, there is a means using cis-1,4-polybutadiene rubber (hereinafter referred to simply as "high-cis BR"). In order to improve the wear resistance by the use of the high-cis BR, means such as the increase of a molecular weight, the betterment of the linearity of a polymer and the reduction of a molecular weight distribution have been heretofore taken. However, as in the case of the formation of the carbon black having the fine particle size, the dispersibility of the carbon black deteriorates noticeably in the blended rubber. Therefore, such an improvement of the wear resistance as to be expected is not observed, and nowadays, the improvement of the wear resistance has reached a substantial limit. Furthermore, the viscosity of the unvulcanized rubber becomes very high, so that workability deteriorates noticeably.
For the purpose of improving the dispersibility of the carbon black and inhibiting the viscosity of the unvulcanized rubber from increasing, there have been heretofore added various kinds of softening agents and plasticizers such as cotton seed oil, soybean oil, pine oil, aroma oil, nathphenic oil and dioctyl phthalate. The addition of the softening agent and the plasticizer can improve the dispersibility and inhibit the increase of the viscosity, but the wear resistance deteriorates conversely.
Furthermore, it has also be attempted to utilize a reactive softening agent and plasticizer with the intention of improving the dispersibility and inhibiting the increase of the viscosity. In this case, the wear resistance can be improved as compared with the employment of the above-mentioned softening agent and plasticizer, but the degree of its improvement is not sufficient and it is the most to stem the deterioration of the wear resistance.
We have intensively investigated in view of this point, and we have suggested a rubber composition having improved static mechanical strength obtained by blending, as a vulcanizing accelerator, a specific unsaturated fatty acid, i.e., an organic unsaturated fatty acid including two or more carbon double bonds in the molecule which includes 5% by weight or more of a conjugated diene acid having at least one conjugated carbon-carbon double bond in its molecule (Japanese Patent Application Laid-open No. 189850/1992).
However, when the specific unsaturated fatty acid is merely added, the improvement effect of dynamic mechanical strength, particularly the wear resistance is poor, though the static mechanical strength can be improved. Thus, a further beneficial suggestion has been desired.